It took eight people to carefully lift the stone slab off the 2.12m by 0.6m outer coffin. Once the lid came off, researchers were startled to discover an inner coffin made of lead.

A hole in the bottom of the casket shows the remains of the occupant's feet.

Grey Friars site director Mathew Morris, of the University of Leicester Archaeological Services (ULAS), said "For me, it was as exciting as finding Richard III. We still don't know who is inside - so there is still a question mark over it."

The lead coffin has been taken to the University of Leicester where tests will be conducted on how to open it without damaging its contents.

"None of us in the team have ever seen a lead coffin within a stone coffin before. We will now need to work out how to open it safely, as we don't want to damage the contents when we are opening the lid," Mr Morris said.

Lead coffins were generally reserved for only the highest-status burials.

The fully intact stone coffin could possibly contain the remains of one of the medieval friary's founders - either Peter Swynsfeld (1272) or William of Nottingham (1330).

Medieval records also suggest a knight named "Mutton", who had also been mayor of Leicester, had been buried there. This is believed to have been Sir William de Moton who died between 1356 and 1362.

However, it is unlikely the identity of the burial will ever be known as few records remain from the era.